488 CHORDATA. 
CHAPTER XXVII. 
THE MAMMALIA. 
Sub-Class I.—Prototheria. 
The Prototheria have only one living order, though there 
are reasons for believing that certain extinct forms of 
mammals may belong to this sub-class. They constitute 
the order AVotheria, whilst the living types comprise the 
order Monotremata. 
Their great importance consists in the fact that they are 
the lowest types of mammals and in many respects they 
form a transition in 
Fig. 332.—DIAGRAM OF THE structure to the rep- 
Fa@TAL MEMBRANES OF EcHIDNA 4s tiles. Like most lowly 
SEEN IN CROSS-SECTION, and primitive forms, 
|, Serosa.. they also have a num- 
Yolk-sac. / Allantois. — ber of very specialised 
features superposed up- 
on their generalised 
organisation. 
We have already 
seen that the division 
into sub-classes is 
based upon the mode 
of reproduction and 
on the comparative 
structure of the repro- 
ductive organs. The 
features of the sub- 
class are therefore as 
follows :— 
The yolk-sac is on the embryo’s left and 
the allantois on the right. - I. REPRODUCTION. 
—It was not till 1884 
that the egg-laying propensities at these mammals were 
definitely discovered. The eggs are much larger than 
